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Search operations for survivors of a scuba diving boat disaster off the California coast were suspended on Tuesday after divers recovered 20 bodies and spotted another four to six trapped in underwater wreckage.

The remains of 11 women and nine men have been transferred to coroner offices following the disaster on Monday, when the 23-metre Conception caught fire and sank with passengers trapped below deck by the roaring blaze.

"Today we will begin the process of mapping DNA profiles of the 20 victims that we have recovered so far, so that they can be compared with family samples," Santa Barbara County Sheriff Bill Brown told a news conference.

The boat had been on a diving excursion around Santa Cruz Island, just west of Santa Barbara in southern California, when disaster struck early Monday.

Five crew members were awake and jumped into the water when flames burst out around 03:15. They were rescued by a nearby pleasure craft.

A total of 39 were on board, officials said, leaving 14 yet to be recovered and presumed dead.

Both a stairwell and an escape hatch leading down to the lower deck where passengers were sleeping appeared to have been blocked by the inferno, Brown said.

On Tuesday, emergency officials will attempt to stabilize the boat so that divers can safely enter it and retrieve remaining victims.

A debris field around half a mile in size will be mapped and searched for additional victims and evidence of what caused the fire.

Mourners gathering at the Santa Barbara docks hung white, red and yellow flowers and wrote messages of condolences on items affixed to a metal fence by the waterfront.

One, written on a pair of blue diving fins, said: "We love you Conception."

'Find out the cause'

The search operation saw three helicopter crews and boats scour a region covering 160 miles for just under 24 hours, but find no signs of survivors, said Coast Guard Captain Monica Rochester.

"It is never an easy decision to suspend search efforts," she said. "We know this is a very difficult time for family and friends of the victims."

The operation will now enter a recovery and investigation phase "to try and determine why this incident occurred," she added.

Mark Hartwig, county fire chief, said authorities were "expending all necessary means to find out the cause and origin of the fire."

Bob Hansen, a sailor on board the nearby pleasure boat that rescued the crew members, told the Los Angeles Times he heard "explosions going off every couple of minutes," which may have been caused by dive tanks exploding.

"It made me feel so helpless," he said.

Rochester, the coast guard captain, said earlier that all the passengers were believed to have been sleeping when the fire broke out.

US news outlets released audio of a distress call in which a crew member on the boat yells, "Mayday! Mayday! Mayday!" and "I can't breathe!"

The Truth Aquatics website said the Conception, listed as having bunks for up to 46 people, had been scheduled to return Monday from a three-day trip after visiting several diving spots around Santa Cruz Island.

It was just 20 metres off the island's northern shore when disaster struck.